1. Field of the Invention
This invention relates to a method for processing metallurgical waste and, more particularly, it pertains to a method for recovering such elemental metals as lead and zinc from smoke and fumes incurred during the refining of a metal such as steel.
2. Description of Prior Art
America's steel industry now produces 33% of its 87,300,000 tons per year output using electric arc furnaces which remelt steel scrap and which have traditionally been referred to as mini-mills, furnishing low and medium cost hot rolled bar mill products to local markets. Unit capacities have increased as has product quality over the past two decades. The mini-mill may now produce more than one million tons of steel annually. Its output must conform to tightly controlled product specifications, and its products are now sold in broad markets.
Electric furnace based steelmaking facilities represent modest capital investment and operating costs when compared to blast furnace basic oxygen furnace operations, but generate large quantities of dusts and fumes. About 500,000 tons of electric arc furnace (EAF) dusts are produced annually in the U.S. These dusts contain undesirable elements which require listing by the Environmental Protection Agency (EPA) as Hazardous Materials in accordance with the Resource Conservation and Recovery Act (RCRA) of 1976. Dusts emanating from a typical electric furnace producing conventional carbon or low alloy steel currently contain up to 35% zinc, up to 4% lead, and may vary from 20% up to 80% iron. Arc furnace dust must now be managed in accordance with a strict set of Federal (and in many cases State) regulations which no longer permit disposal in simple landfills, thus imposing substantial added constraints and costs upon arc furnace producers.
The steel industry has gradually increased its use of the electric arc furnace for primary hot metal production as marginal blast furnace complexes continue to be phased out. Since 1975, arc furnace production has increased from 23 million tons per year (19%) to 29 million tons (33%) in 1985 and this trend is expected to continue for the foreseeable future. Increased use of galvanized steels has tended to increase the zinc content and this trend is also expected to continue to increase as auto producers and other manufacturers move increasingly to longer life, corrosion resistant products.
Extremely fine dusts, with particle sizes ranging from 0.1 to 1.0 microns, are formed by metal vaporization in the electric arc furnace and subsequent reaction with entrained oxygen in the furnace and air pollution control system. The initial vaporization occurs at the ultra-high temperature arcing zone, due to boiling in the molten bath as the melt is being refined, and because the coating materials on the charge scrap, i.e. zinc, lead, and cadmium, are highly volatile at the temperature of molten steel. Due to the conditions at their formation, individual dust particles tend to be complex multicomponent metal oxides. As such, they have been listed by the EPA as hazardous wastes due to the leachability of such contained toxic tramp elements as lead, cadmium, and chromium. However, they do not lend themselves to conventional processing techniques which would render these dusts delistable.
As hazardous wastes, arc furnace dusts may no longer be disposed of in conventional landfills after simple wetting to prevent wind dispersal. Recent RCRA amendments have imposed increasingly restrictive controls on disposal. In 1984, the EPA required disposal of EAF dusts in secured sites, retrofitted with double liners and systems for collection and removal of leachates. More recently, regulations have banned open landfill disposal altogether, requiring containerization with attendant costs which could considerably exceed $100 per ton.
Rapidly escalating constraints and disposal costs have prompted a number of technology development efforts aimed at treatment of the arc furnace dusts so that they may be delisted and disposed of in a conventional manner. Efforts are underway to develop regional as well as on site dust processing systems, and range from chemical leachant systems for removal of toxic elements to reinjection of dusts into arc furnaces for the purpose of increasing zinc, lead and cadmium concentrations to levels acceptable as feedstocks for established non-ferrous metal producers.
Electric arc furnace baghouse dusts are oxidic in nature and contain the following ranges of composition:
______________________________________ Iron Oxides 45.-89.% Manganese Oxide 3.-6.% Silicon Oxide 3.-0.9% Chromium Oxide 0.1-0.9% Nickel Oxide 0-0.2% Carbon 0-0.4% Copper Oxide 0-0.4% Zinc Oxide 13.9-33.5% Lead Oxide 1.2-4.1% Cadmium Oxide 0-0.2% Tin Oxide 0-0.1% Magnesium Oxide 1.-7.% Calcium Oxide 2.6-14.% Aluminum Oxide 0.2-0.7% Alkali Salts & 1.7-7.8% Sulfates ______________________________________
A number of carbothermic reduction systems have been investigated and proposed for operation typically at temperatures of 2600.degree.-3000.degree. F. Given sufficient carbon and mixing; reduction of the non-slag forming oxides proceeds rapidly providing a metallic melt consisting of iron, manganese, nickel, copper and equilibrium fractions of silicon and chromium. Many of the non-ferrous metals are in the vapor state at the operating temperature. These include zinc, lead and small amounts of cadmium; they may be condensed and collected as liquid metals for separately casting into ingots. (Zinc and lead have less than 1% mutual solubility). The slag formers - oxides of calcium, aluminum, chromium, magnesium and silicon, as well as the alkali and sulfate salts form as a liquidus slag on the melt which may be tapped conventionally. Depending on its chrome concentration and structure, the slag can be delisted and granulated for highway construction.
The advantage of the systems briefly outlined above lies in their ability to produce high valued metallic products (zinc, lead, and the ferrous melt) while at the same time solving a serious national disposal problem. Such systems are segregated into on site and regional processing facilities. The on site system is particularly desirable as it allows the steelmaker to "control his own destiny" with regard to disposal, while at the same time saving a large transportation and disposal fee (now projected to exceed $100 per ton). Such small on site systems suffer, however, from high unit costs owing to their generally small size. A typical 400,000 ton per year mini-mill would produce about 25 to 30 pounds of dust per ton of steel or 5,000 to 6,000 tons of dust annually. Regional facilities, handling 50,000 to 100,000 tons per year have also been proposed and one has been built in Sweden. These larger units offer the advantage of lower unit costs but require transportation charges for supply of the dust.
All the above units employ plasma systems to provide the required energy. Such systems are characterized by utilization of an electric arc discharge for either direct heating of the dust and carbon source or heating of a carrier gas which is in turn used for heat transfer to the reactants. Plasma systems have the advantage of very high temperature operation providing extremely rapid heat transfer while at the same time providing the capability to super heat a highly reducing gas stream for efficient process chemistry.
The drawbacks to plasma systems, however, are expensive power supplies (owing to the negative impedance characteristics of plasmas) and the inherent requirement to handle large volumes of gas within the system. This generally involves recycling the gas to the plasma torch. Large scrubbers, driers, compressors, piping and flow components--are therefore required, entailing major capital, operating and maintenance expenses for the system manager.
A low capital cost system is required into which dust, carbon, and energy can be supplied and steel, nonferrous metals, slag and a fuel gas can be produced with no requirement for troublesome and expensive recirculation, and which can operate controllably using low cost power supplies. The latter requirement could be satisfied by a system having a positive impedance load characteristic.
A typical EAF dust generated during production of carbon steel, has the following analysis * (when adjusted to oxidic constituents):
______________________________________ Al.sub.2 O.sub.3 0.37% (by weight) CaCl.sub.2 1.57 CaF.sub.2 1.85 CaO 4.80 CsSO.sub.4 1.53 CdO 0.03 Cr.sub.2 O.sub.z 0.56 CuO 0.21 Fe.sub.2 O.sub.3 2.43 Fe.sub.3 O.sub.4 48.51 MgO 6.73 MnO 3.87 Na.sub.2 O 0.83 NiO 0.09 PbO 1.62 SiO.sub.2 5.30 SnO 0.07 ZnO 19.17 C 0.44 ______________________________________ *Excerpted from "Electric Arc Furnace Dust", D. R. MacRae Center for Metals Production Report 852, May 1985
This dust can be reacted with a carbon source such as coke breeze or preferably bituminous coal in a plasma fired furnace, such as the Mesabi Metal Reactor (described in U.S. Pat. No. 4,571,259), and the coke filled shaft furnace, (described in U.S. Pat. No. 4,530,101). While requirements may vary slightly among the several above-referenced systems, all will produce a melt, slag and non-ferrous metal vapor having the following expected compositions:
______________________________________ Melt Non-Ferrous ______________________________________ Chromium 0.9% (weight) Cadmium 0.2% (weight) Copper 0.4 Lead 8.9 Manganese 7.2 Zinc 90.9 Nickel 0.2 Tin 0.1 Carbon 0.5 (may be 3.0-4.5) Iron balance ______________________________________ Slag ______________________________________ Alumina 1.5% (weight) Calcium Chloride 6.3* Calcium Fluoride 7.4* Calcia 19.3 Calcium Sulfate 6.1 Magnesia 27.0 Sodium Oxide 3.3* Silica 29.1** ______________________________________ The approximate material balance is: ______________________________________ Input Dust 1000. lbs Silica 39. lbs Bituminous Coal 235. lbs Products Melt 372. lbs Slag 385. lbs Zinc 146. lbs Lead 15. lbs Gas (75% CO;25% H.sub.2) 315. lbs ______________________________________ *For analysis purposes, all Chlorine & Fluorine are shown to report to Calcium. In practice, they will also report to Sodium & Magnesium in equilibrium amounts. **Silica is added to adjust the basicity ratio of the slag.
The theoretical energy requirements for providing sensible heat and reaction energy to provide products at a temperature of 1900.degree. K. (2960.degree. F.) is 654. kwhr per 1000 pounds of dust (1308 kwhr/ton).
In all three plasma fired systems described above, off gas would be recycled to the plasma torches and superheated to an enthalpy of about 5 kwhr/Nm.sup.3. Such torches operate at an efficiency of about 80%. The enthalpy of the above gas mixture (75% CO/25% Hz) at the specified 1900.degree. K. is 0.64 kwhr per Nm.sup.3. Assuming no furnace losses, the energy requirement for the system described above is ##EQU1##
This estimate compares very well with the reported value for the SKF system of 1890 kwhr/ton (MacRae). From an energy consumption standpoint, it would be highly desirable to develop a system which avoids the gas recycle and simultaneously operates at increased heating efficiency, thus approaching an energy requirement of 1308 kwhr per ton, rather than 1875 kwhr/ton.